Evaluating method for NOx eliminating catalyst, an evaluating apparatus therefor, and an efficiency controlling method therefor

ABSTRACT

In order to prevent a catalyst for an internal combustion engine from decreasing efficiency by deterioration after long time use of the catalyst, the decreased efficiency of the catalyst is determined, and the internal engine is controlled based on results of the determination so as to maintain high efficiency of the catalyst. The catalyst is installed in an exhaust pipe of the engine. Sensors for detecting conditions of exhaust gas both at upstream side and downstream side of the catalyst are provided, respectively. As for the sensor, for example, an oxygen sensor of which output varies stepwise at λ=1, or a sensor of which output varies in proportion to air-fuel ratio can be used. Detected values of the sensors are taken into a control unit, eliminating efficiency and deteriorating degree of the catalyst are estimated by comparison of the detected values, and the engine is controlled so that the eliminating efficiency becomes maximum. In accordance with the present invention, a preferable exhaust gas cleaning characteristics of the catalyst can be maintained because the decreased efficiency of the catalyst is determined exactly and the engine is controlled so as to prevent the catalyst from decreasing efficiency.

This application is a continuation of application Ser. No. 08/917,819,filed Aug. 27, 1997, now abandoned, which is a division of applicationSer. No. 08/264,068, filed Jun. 22, 1994, now U.S. Pat. No. 5,693,877.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an evaluating method for NO_(x)eliminating catalyst provided in an exhaust gas system of internalcombustion engines, an evaluating apparatus therefor, and an efficiencycontrolling method therefor. The above described NO_(x) eliminatingcatalyst means essentially a catalyst for eliminating NO_(x) componentsin exhaust gas for cleaning the exhaust gas in the exhaust gas system ofthe internal combustion engine.

(2) Description of the Prior Art

As for a method for estimating efficiency decrease and deterioration ofNO_(x) eliminating catalyst, a method has been proposed in JP-A-4-265414(1992), for example, wherein, taking a travelling distance of anautomobile as a parameter, efficiency of NO_(x) eliminating catalyst inthe automobile is deemed to be sufficiently deteriorated when thetravelling distance exceeds a designated value.

Further, a method for increasing an amount of hydrocarbon (called as HChereinafter) when the HC used for NO_(x) eliminating catalyst is deemedto be insufficient has been proposed in JP-A-3-229914 (1991).

However, there has been a problem that the prior art described above areunable to evaluate the catalyst correctly because they use a travellingdistance as a parameter for evaluating the catalyst indirectly.

Furthermore, regarding to the prior art controlling the amount of HC,there has been a problem that the prior art can not control the amountof HC correctly because it controls the amount of HC irrelevant to theevaluating result of the catalyst deterioration.

SUMMARY OF THE INVENTION

(1) Objects of the Invention:

In order to solve the above described problems, the object of thepresent invention is to provide a preferable evaluating method forNO_(x) eliminating catalyst, a preferable evaluating apparatus forNO_(x) eliminating catalyst, and a preferable efficiency evaluatingmethod for NO_(x) eliminating catalyst.

(2) Methods for Solving the Problems:

One of features of the present invention is essentially in an evaluatingmethod of NO_(x) eliminating catalyst for eliminating NO_(x) componentsin exhaust gas, which is characterized in evaluating the NO_(x)eliminating catalyst by comparing physical parameters of specifiedexhaust gas components both at an upstream side and a downstream side ofthe NO_(x) eliminating catalyst.

Other one of the features of the present invention is essentially in anevaluating apparatus for NO_(x) eliminating catalyst comprising;

(a) a base body composed of ion conductive solid electrolyte,

(b) platinum electrodes provided at both sides of the base body puttingthe base body between,

(c) diffused resistors covering the both electrodes respectively,

(d) exhaust gas inlet paths for flowing exhaust gas at upstream side ofthe NO_(x) eliminating catalyst to one of the diffused resistors andflowing exhaust gas at downstream side of the NO_(x) eliminatingcatalyst to the another of the diffused resistors respectively,

(e) an output portion which supplies an electrical signals outputgenerated between the both platinum electrodes to a postfixed evaluatingmeans.

Furthermore, other one of the features of the present invention isessentially in an efficiency controlling method for NO_(x) eliminatingcatalyst for eliminating NO_(x) components in exhaust gas, which ischaracterized in including the steps of evaluating the NO_(x)eliminating catalyst first and subsequently elevating a temperature ofthe NO_(x) eliminating catalyst or increasing an amount of HC.

In the above evaluating apparatus, oxygen concentration in exhaust gasat downstream of the NO_(x) eliminating catalyst increases by reducingeffect of the NO_(x) eliminating catalyst. Therefore, efficiency and adeterioration degree of the NO_(x) eliminating catalyst can be estimatedby comparing physical parameters of specified gas components at upstreamside and that at downstream side of the NO_(x) eliminating catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagrammatic view showing a total system of thepresent invention,

FIG. 2(A) and FIG. 2(B) are graphs showing characteristic of catalystconversion efficiency,

FIG. 3(A) and FIG. 3(B) are graphs showing characteristic of catalystconversion efficiency,

FIG. 4(A) and FIG. 4(B) are schematic illustrations for explaining aprinciple of eliminating NO_(x) by the catalyst relating to the presentinvention,

FIG. 5(A) and FIG. 5(B) are schematic illustrations for explaining aprinciple of detecting deterioration of the catalyst relating to thepresent invention and FIG. 5(C) is a graph showing oxygen concentrationbefore and after the catalyst by way of output voltage of oxygensensors,

FIG. 6(A) is a schematic illustration for explaining a detecting methodrelating to the present invention,

FIGS. 6(B), 6(C), and 6(D) are graphs for explaining the detectingmethod shown in FIG. 6(A),

FIG. 7(A) is a flow diagram relating to the present invention,

FIG. 7(B) is a schematic graph for explaining the flow diagram shown inFIG. 7(A),

FIG. 8 is a flow diagram relating to the present invention,

FIG. 9 is a flow diagram relating to the present invention,

FIG. 10(A) is a schematic illustration for explaining another detectingmethod relating to the present invention,

FIGS. 10(B), 10(C), and 10(D) are graphs for explaining the detectingmethod shown in FIG. 10(A),

FIG. 11 is a schematic illustration of a detecting apparatus relating tothe present invention,

FIG. 12 is a flow diagram for controlling the detecting apparatus shownin FIG. 11,

FIG. 13 is a schematic illustration of another detecting apparatusrelating to the present invention,

FIG. 14 is a flow diagram for controlling the detecting apparatus shownin FIG. 13,

FIG. 15(A) and FIG. 15(B) are schematic illustrations for explaininganother detecting method relating to the present invention,

FIG. 16(A) is a cross section of a detecting sensor used in thedetecting method shown in FIG. 15

FIGS. 16(B) and 16(C) are graphs for indicating characteristics of thedetecting sensor shown in FIG. 16

FIG. 17 is a flow diagram for control in the detecting method shown inFIG. 15(A),

FIG. 18 is a schematic illustration for explaining another detectingmethod relating to the present invention,

FIG. 19 is a schematic illustration for explaining another detectingmethod relating to the present invention,

FIG. 20 is a schematic illustration for explaining another detectingmethod relating to the present invention,

FIG. 21 is a simplified diagrammatic view showing a total system ofanother embodiment of the present invention,

FIG. 22 is a simplified diagrammatic view showing a total system ofanother embodiment of the present invention,

FIG. 23 is a flow diagram for control in the detecting methods shown inFIG. 21 and FIG. 22,

FIG. 24 is a flow diagram for control in the detecting methods shown inFIG. 21 and FIG. 22,

FIG. 25 is a flow diagram for control in the detecting methods shown inFIG. 21 and FIG. 22,

FIG. 26(A) is a schematic illustration for explaining another detectingmethod relating to the present invention,

FIGS. 26(B) and 26(C) are graphs for explaining the detecting methodshown in FIG. 26(A),

FIG. 27 is a flow diagram for control in the detecting method shown inFIG. 26(A),

FIG. 28(A) is a schematic illustration for explaining another detectingmethod relating to the present invention,

FIGS. 28(B) and 28(C) are graphs for explaining the detecting methodshown in FIG. 26(A),

FIG. 29 is a flow diagram for control in the detecting method shown inFIG. 28(A).

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiment 1

A simplified diagrammatic view showing a total system of an embodimentof the present invention is illustrated in FIG. 1.

A catalyst 3 is connected to an exhaust gas pipe 2 of an engine 1. Inthe catalyst 3, lean NO_(x) catalysts 4 for eliminating NO_(x) under alean air-fuel ratio condition, and a three way catalyst or an oxidecatalyst 5 for eliminating NO_(x), CO, and HC under a theoreticalair-fuel ratio condition. The catalyst 3 is composed in a manner so asto switch the above two kinds of catalysts by switching valves 6depending on an operating condition of the engine. As for the leanNO_(x) catalyst, a copper-zeolite catalyst containing metals, forexample, can be used. However, the above exemplified catalyst generallyhas such a characteristics that the catalyst deteriorates itself under ahigh temperature or a rich air-fuel ratio condition. Therefore,bypassing the lean NO_(x) catalyst are preferable in some cases, forexample, when in a power driving condition, in a warming up condition atstarting, and so on. Accordingly, the switching valves 6 are provided.Under a lean operation condition, the switching valves 6 are closed soas to supply exhaust gas to the lean NO_(x) catalyst. On the other hand,under a rich air-fuel mixture condition such as a power drivingcondition and a warming up condition at starting, the switching valves 6are opened so as to supply the exhaust gas to the three way catalyst orthe oxide catalyst provided at downstream side. In order to determineNO_(x) conversion efficiency of the lean NO_(x) catalyst, sensors 7, 8,for detecting the exhaust gas conditions are provided at, for instance,before and after the catalyst 3. As for the sensors 7, 8, for example,an oxygen sensor of which output changes stepwise by an excess airfactor λ=1, and an air-fuel ratio sensor of which output changesproportionally to the excess air factor can be usable. Detected valuesat the above two sensors are taken into a control unit 9, and theconversion efficiency or a deteriorating degree of the catalyst isdetermined by comparing the values. Air taken through an air cleaner 10is flowed into a collector 14, after being measured its amount by an airsensor 11, via a throttle 13 which is driven by an electric motor 12.Subsequently, the air passes through an independent intake pipe 15 andis taken into the engine 1. At an intake port portion 16, a bypassingpath 17 and a dividing valve 18 for generating swirls are provided. In alean operating region, it is necessary to form swirls in a combustionchamber for improving combustion. Accordingly, in the above case, thedividing valve 18 is closed so that the air flows through the bypassingpath 17. Drift is caused in the air, and consequently, swirls aregenerated in the combustion chamber. Fuel is supplied through a fuelinjector 19. The air-fuel mixed gas is ignited by an ignition plug 20. Acrank angle detector 21 for determining rotation of the engine shaft peran unit time is provided at a crank shaft 22.

FIG. 2(A) and FIG. 2(B) are graphs indicating characteristics ofefficiency of the lean NO_(x) catalyst to convert NO_(x) to N₂. FIG.2(A) is a graph indicating a relationship between catalyst temperatureand conversion efficiency. The FIG. 2(A) reveals that the catalyst hashigh efficiency in a certain range of temperature, and that the range oftemperature of high efficiency shifts toward a higher temperature rangein accordance with deteriorating the catalyst. The above described shiftof maximum efficiency temperature is indicated graphically in FIG. 2(B).FIG. 2(B) reveals that the maximum efficiency temperature increases inaccordance with increase of travelling distance of the automobile, thatis, with increase of the deteriorating degree of the catalyst. Arelationship between an amount of HC in the exhaust gas and theconversion efficiency is shown in FIG. 3(A). FIG. 3(A) indicates thatthere is an optimum amount of HC for obtaining maximum conversion ratiocorresponding to an amount of NO_(x), and a higher conversion ratio canbe achieved by controlling the amount of HC so as to be the aboveoptimum amount. However, the optimum amount of HC for achieving themaximum conversion ratio alters depending on deterioration of thecatalyst as shown in FIG. 3(B). Therefore, it is revealed thatdetermining deterioration of the catalyst regularly and changing theamount of HC corresponding to the deterioration of the catalyst arenecessary.

Next, methods for determining conversion ratio of catalyst anddeterioration of the catalyst are explained hereinafter. As shown inFIG. 4(A), exhaust gas components, which relates to reducing reactionsof NO_(x), at an engine side 30 of a lean NO_(x) catalyst 4 which isinstalled at inside of an exhaust pipe 3 are NO_(x), HC, and O₂. In FIG.4(B), molecules of gases are schematically illustrated. Nitrogen,non-combustion hydrocarbon, and oxygen are indicated by N, HC, and O,respectively. On a catalyst, HC reacts with oxygen to form anintermediate product (which is indicated by HC surrounded by arectangular frame) which effects to NO_(x) to decompose to N₂.Accordingly, the exhaust gas at downstream side 31 of the catalystcontains N₂, H₂O, and CO2 because of reducing the NO_(x). In this case,oxygen concentration alters before and after the catalyst as shown inFIG. 4(B). Therefore, as one of detecting methods of change in theconversion ratio of the NO_(x), a method to detect oxygen concentrationbefore and after the lean NO_(x) catalyst can be proposed.

A principle of detecting oxygen concentration is illustrated in FIGS.5(A), 5(B), and 5(C). Referring to FIG. 5(A), sensors 7, 8, fordetecting oxygen concentration are installed before and after thecatalyst, respectively. The sensor 7 for detecting oxygen concentrationis composed of platinum electrodes 33 a, 33 b, which are attached atboth sides of a zirconia solid electrolyte 32 respectively. A diffusedresistor 34 which regulates gas diffusion is formed on an exhaust gasside of the electrode 33 a attached at the exhaust gas side of theelectrolyte. The sensor 7 is connected to a casing such as an exhaustpipe 3 and the electrode 33 a as a grounding. In this case, when aspecified voltage is applied to another electrode, generated electriccurrent becomes proportional to oxygen concentration at the exhaust gasside. That means, the oxygen concentration can be determined bymeasuring a value of the electric current. Composition and operation ofthe sensor 8 is the same as that of the sensor 7.

Components of exhaust gas are schematically illustrated in FIG. 5(B).Before reactions at the catalyst, nitrogen, HC, and oxygen coexist asshown in (a). However, HC is almost completely oxidized by a catalyticreaction of platinum on the electrode 33 a of the sensor 7. Therefore,detected oxygen concentration is less as much as an amount consumed forthe oxidation reaction as shown in (b) as the amount surrounded by onedot chain line in a rectangular frame. Further, because the HC is almostcompletely reacted and oxygen is generated by decomposition of NO_(x) bya reducing reaction at downstream side of the catalyst 4, oxygenconcentration detected at the downstream side of the catalyst 4increases more than that of the (b) condition as shown in (c) as theamount surrounded by one dot chain line in a rectangular frame.Accordingly, as a comparison of the amounts of oxygen indicated byrectangular frames in (b) and (c) reveals, detected oxygen concentrationbefore and after the catalyst 4 differ each other. Therefore, signalsfrom the oxygen sensors before the catalyst and the oxygen sensor afterthe catalyst indicate different values each other as shown in FIG. 5(C).

A principle of detecting deterioration or conversion ratio isillustrated in FIGS. 6(A), 6(B), 6(C), and 6(D). FIG. 6(A) illustratesschematically an outline of an apparatus therefor. A specified voltageis applied to the electrode 33 b of the sensors 7, 8. At that time,voltage drops V1, V2 of electric current flown through fixed resistorsR1, R2 are detected by differential amplifiers 36, 37. These V1, V2 arevalues of electric current flown through the solid electrolyte 32 ofrespective sensors 7, 8, that means, detected oxygen concentration.Further, a difference of the V1 and V2 is detected again by adifferential amplifier 39. The difference (V1−V2) is a value related todeterioration. The above described sensor changes its output dependingon oxygen concentration as indicated in FIG. 6(B), and accordingly,difference of oxygen concentration before and after the catalyst 4 canbe detected. The difference of output voltages are shown in FIGS. 6(C)and 6(D). The V2 is higher than V1 as much as difference of oxygenconcentration. The output V1 of the sensor 7 before the catalyst can beused concurrently for controlling air-fuel ratio. Naturally, the outputV2 of the sensor 8 after the catalyst also can be used concurrently forcontrolling air-fuel ratio.

In the present embodiment, the difference of V1 and V2 is detected by adifferential amplifier, but the difference can be obtained by taking V1,V2 into a microcomputer after converted by an analog-digital convertorand calculating the difference by a calculating process. A flow chartfor the calculation is explained hereinafter.

First, a flow chart for controlling air-fuel ratio is shown in FIG.7(A). The V1 is measured at step 100, and a target output V_(ref) of thesensor corresponding to a target air-fuel ratio (A/F) as shown in FIG.7(B) is retrieved from a map of engine rotation N and load at step 110.The V1 is compared with the V_(ref) at step 120. If the V1 is largerthan V_(ref), the air-fuel ratio at the time is judged as in a leanerside than the target air-fuel ratio, and an amount of fuel injection Tiis increased at step 130 so as to shift the air-fuel ratio to a richside. If the V1 is smaller, the air-fuel ratio at the time is judged asin a richer side than the target air-fuel ratio, and an amount of fuelinjection Ti is decreased at step 140 so as to shift the air-fuel ratioto a lean side. As described above, T is determined and output to a fuelinjection valve 19 at step 150. In a manner as described above,controlling of air-fuel ratio can be realized by using sensors fordetecting conversion ratio or deterioration.

Next, a flow chart for detecting deterioration of catalyst is shown inFIG. 8. At step 210, V1 and V2 are measured and a difference of the V1and the V2 is calculated. If the difference is smaller than a specifiedvalue at step 220, oxygen increase by reducing reaction of N₂ by thecatalyst is judged as small at step 230 and the catalyst is estimated tobe deteriorated. That is, the catalyst is judged to be deteriorated anda deterioration degree is indicted. The larger difference of oxygenconcentration before and after the catalyst, that is, the largerdifference between V1 and V2 means a stronger reducing reaction of thecatalyst and indicates that the catalyst is not deteriorated yet.

A deterioration judging method improved in accuracy is indicated in FIG.9. The difference between V1 and V2 is calculated at step 300, andcatalyst temperature Tc or exhaust gas temperature is judged whether thetemperature is in a specified range or not at step 310. Because theconversion ratio of the catalyst varies depending on temperature asshown in FIG. 2(A), and there may be a possibility to judge the catalystto be deteriorated erroneously when the catalyst temperature changes.Therefore, the deterioration judgement of the catalyst is performed onlyat the time when the catalyst temperature exists in a specified range.Furthermore, the above described method is effective in a point thattemperature dependency of characteristics of the sensor can be ignored.The deterioration judgement on the catalyst after the catalysttemperature is confirmed to be in a specified range is performed by thesame steps as shown in FIG. 8. That is, when a difference between V2 andV1 obtained at step 320 is lower than a reference value, the catalyst isjudged as deteriorated at step 330, and a deteriorating degreecorresponding to the difference between V2 and V1 is indicated at step340.

Next, another detecting method is shown in FIGS. 10(A), 10(B), 10(C),and 10(D). A sensor used in the present method is an oxygen sensorhaving a non-linear output characteristics corresponding to oxygenconcentration (air-fuel ratio) as shown in FIG. 10(B). In a case usingsuch a sensor as above described, a simple diffused film 42 can be usedsufficiently as the diffused film provided at surface of an electrode 40a in an exhausted gas side of the each sensor 7, 8. Concretely saying, athinner diffused film than the diffused film 34 shown in FIG. 5 can beused. The apparatus shown in FIG. 10(A) uses such oxygen sensors as theones having two values type output. With the above sensor, the electrode40 a at the exhaust gas side is connected to a ground, and voltages(electromotive force) V1, V2 generated at the other electrode 40 b aremeasured. Based on the difference between the voltages of the twosensors, the deteriorating degree is detected. Concretely saying, V1 andV2 are measured when the exhaust gas is more lean than a theoreticalair-fuel ratio, and the difference between the outputs is determined bya differential amplifier 44. However, the difference can be calculatedby taking the outputs into a microcomputer as previously explained.Based on the difference, a deteriorating degree of the catalyst isestimated. Examples of the outputs, V1, V2, are illustrated in FIGS.10(C) and 10(D). The output after the catalyst, V2, has less value thanthe output before the catalyst as shown in FIGS. 10(D) and 10(C),respectively, because the exhaust gas after the catalyst contains moreoxygen relatively than the exhaust gas before the catalyst. Thedifference between V1 and V2 becomes an index indicating thedeteriorating degree of the catalyst.

Embodiment 2

Further, another embodiment of the present invention is shown in FIG.11. In the present embodiment, a prefixed catalyst 53 and a postfixedcatalyst 54 in a downstream side were arranged in series, and threesensors, 50, 51, 52 were installed. Efficiency and a deterioratingdegree of the prefixed catalyst 53 are determined by the methodexplained in the previous embodiment using the sensors 50, 51.Efficiency and a deteriorating degree of the postfixed catalyst 54 aredetermined by the method explained in the previous embodiment using thesensors 51, 52, or sensors, 50, 52. In accordance with the abovedescribed arrangement, deterioration diagnosis of a complex catalystsystem becomes possible. As for the catalyst, a NO_(x) reducing catalystis used for the prefixed catalyst 53 and a three way catalyst or anoxidizing catalyst is used for the postfixed catalyst 54. In this case,a deteriorating degree of the NO_(x) reducing catalyst 53 is determinedby comparing detected outputs of the sensors 50 and 51 as previouslyexplained. A deteriorating degree of the postfixed catalyst 54 can bedetermined using signals from the sensors 51 and 52, or sensors 50 and52. The signals from the sensors 50, 51, 52 are taken into amicrocomputer 9, and are processed for calculation. A controlling flowchart in this case is shown in FIG. 12.

The NO_(x) reducing catalyst 53 is effective as a NO_(x) eliminatingcatalyst in a lean operating region, and accordingly, judging firstwhether the catalyst is within the lean operating region at step 400.When the catalyst is judged as within the lean operating region, adiagnosis mode starts. That is, signals V1, V2 from the sensors 50, 51before and after the catalyst 53 respectively are determined at step410, and the diagnosis of deterioration is performed at step 420. Forthis diagnosis, flow charts shown in FIGS. 8 and 9 are used.Subsequently, a deteriorating degree of the catalyst is indicated atstep 430.

In a case of the postfixed catalyst, a deteriorating degree is judgedwhen the catalyst is within an operating region with an excess airfactor λ=1 at step 440. Signals V1, V3 from the sensors 50, 52 in theabove described operating region are determined at step 450, and thediagnosis of deterioration is performed at step 460 based on the abovesignals. In this case, the deterioration of the catalyst can also bejudged by measuring outputs V2, V3 from the sensors 51, 52 in a samemanner.

As explained above, diagnosis of efficiency or deterioration of acomplex catalysis system using a plurality of catalysts can be performedpreferably when each of the catalysts is within an operation region.

Embodiment 3

Furthermore, another embodiment of the present invention is illustratedin FIG. 13. In this case, a plurality of catalysts are arranged inparallel. Catalysts 55, 56 are arranged in parallel, and a catalyst towhich exhaust gas is flowed is selectively alterable depending on itsoperating condition by switching valves 57A, 57B driven by an actuator58 which is operated by loads or electric power. For instance, when theswitching valve 57A is open so as to flow the exhaust gas to thecatalyst 56, the switching valve 57B is close so as not to flow theexhaust gas to the catalyst 55. In this case, efficiency ordeterioration of the catalyst 56 must be judged based on signals fromthe sensors 58, 59 when the operating condition is such that thecatalyst 56 must work. When the switching valves 57A, 57B are rotated soas to flow the exhaust gas to the catalyst 55, the exhaust gas flow tothe catalyst 56 is stopped. In this case, efficiency or deterioration ofthe catalyst 55 must be judged based on signals from the sensors 58, 59when the operating condition is such that the catalyst 55 must work.Operation of the actuator 58, intake of signals from the sensors 58, 59,and processing are performed by a microcomputer 9.

A flow chart for the above processing is shown in FIG. 14. A case whenone of the catalysts shown in FIG. 13 is a NO_(x) reducing catalyst, andanother catalyst is a three way catalyst or an oxidizing catalyst isexplained hereinafter. First, the operating condition is judged whetherwithin a lean operating region or not at step 500. When the operatingcondition is within the lean operating region, the switching valve 57Ais operated so as to supply exhaust gas to the NO_(x) reducing catalystat step 510. Subsequently, after the operating condition is adequatelyestablished, signals from the sensors 58, 59 are measured at step 520,diagnosis of deterioration is performed on the NO_(x) reducing catalystat step 530, and results of the diagnosis are indicated at step 540.When the operating condition is without the lean operating region, theswitching valve 57A is closed and the switching valve 57B is openedreversely so as to flow the exhaust gas to the three way catalyst atstep 550. Subsequently, the operating condition is judged whether it iswith a theoretical air-fuel ratio λ=1 or not at step 560. When λ=1,signals from the sensors 58, 59 are measured at step 570, diagnosis ofdeterioration is performed on the three way catalyst at step 580, andresults of the diagnosis are indicated at step 540.

Embodiment 4

Furthermore, another embodiment of the present invention is shown inFIG. 15. In the present embodiment, a sensor 61 is so composed that theexhaust gas at upstream side of the catalyst 60 is supplied to one planeof the sensor 61 and the exhaust gas at downstream side of the catalyst60 is supplied to another plane of the sensor 61 in order to determine adifference of oxygen concentration in the exhaust gas at upstream sideand the exhaust gas at down stream side with only one sensor. In thiscase, almost exhaust gas in an exhaust pipe 3 flows to the catalyst 60.However, a small portion of the exhaust gas flows through a path 62 to achamber 63 which is provided at one side of the sensor 61. The exhaustgas further flows through a path 64 by a sucking effect of a Venturi 65which is provided at upstream side of the catalyst. On the other hand,the exhaust gas after the catalyst 60 is led into an exhaust pipe sideof the sensor 61. A structure of the sensor 61 is indicated in FIG.15(B). In FIG. 15(B), the exhaust gas at upstream side of the catalystis led to left side of the sensor, and the exhaust gas at downstreamside of the catalyst is led to right side of the sensor. The sensor iscomposed of a zirconia solid electrolyte 66 having platinum electrodes67 d, 67 b at both sides, and porous protecting films 68 a, 68 b areprovided outside each of the electrodes. The both sides of the solidelectrolyte 66 have catalytic effects and can oxidize HC. In accordancewith the structure, the solid electrolyte 66 can determine residualoxygen concentration after the above oxidation. In this case, the solidelectrolyte, which is a kind of oxygen concentration cell, is preferablebecause only a difference of oxygen concentrations between theconcentration at one side and at the other side of the solid electrolyteis required. Furthermore, if the electrode 67 b at one side is connectedto ground, an electric potential measured at the other electrode 67 aindicates the difference of oxygen concentration. The measured electricpotential is taken into a microcomputer 9, and is processed.

More detailed structure of the sensor 61 is indicated in FIG. 16(A). Thesensor is arranged in a protecting tube 70. The electrode 67 b atdownstream side of the exhaust gas from the catalyst is connected toground by the protecting tube 70 through a wire printed on an insulator71. The other electrode 67 a at upstream side of the exhaust gas to thecatalyst is led to outside through a connector portion 69. The sensorbody itself is screwed and fixed at the exhaust pipe 72. In accordancewith the above described composition, the exhaust gas before and afterthe catalyst can be introduced to each of the sides of the sensor. Thesensor, a kind of oxygen concentration cell, has a characteristics shownin FIG. 16(B) depending on the difference of oxygen concentrationsbetween those at each side of the sensor. During a period when thecatalyst is not deteriorated, oxygen concentration in the downstream ofthe exhaust gas from the catalyst is large because oxygen in NO_(x) isreduced to oxygen molecules as shown in FIG. 5. When the difference ofoxygen concentration between those at each side of the sensor is large,an output voltage of the sensor is low as shown by a point (a) in FIG.16(B). On the contrary, when the difference of oxygen concentrationbetween those at each side of the sensor is small, the output of thesensor increases because of generating electromotive force in the solidelectrolyte 66 as shown by a point (b) in FIG. 16(B). Accordingly, theoutput from the sensor increases with elapsing time as shown in FIG.16(C). Therefore, deterioration of the catalyst can be detected bymeasuring the output of the sensor. As described above, when thedeterioration of the catalyst is detected by only one sensor, accuracyof the detection can be improved because temperatures of detecting sidesof the sensor are identical and temperature dependency of the sensor canbe eliminated.

A flow chart for detecting deterioration of catalyst is shown in FIG.17. First, the operating condition is judged whether within a leanoperating region or not at step 600. When the operating condition iswithin the lean operating region, further, the exhaust gas temperatureTg is judged whether it is within a specified range or not at step 610.When the temperature is within the specified range, the sensor is beingactivated, and temperature dependency of the catalyst can be eliminated.As described above, no heater for the sensor becomes necessary bychoosing temperature range necessary for sensor activation as for thespecified temperature range. Subsequently, an output from the sensor ismeasured at step 620, and a deterioration degree is judged at step 630.This judgement can be performed only by judging the output from thesensor is over or below a reference value. A deteriorating degree isjudged and indicated at step 640.

Embodiment 5

Further, another embodiment of the present invention is shown in FIG.18. Under a lean operating condition, exhaust gas flows into a NO_(x)reducing catalyst 4 by closing a switching valve 6, and subsequently theexhaust gas flows into a three way catalyst 5 which is located atdownstream side. In this case, the switching valve 6 is made in a mannerto leak a small amount of gas so that the exhaust gas at upstream sideflows into both a sensor 73 as shown in FIG. 16(A) and a catalyst 4. Theexhaust gas at downstream side of the catalyst 4 flows through anotherside of the sensor 73. Signals from the sensor are taken into amicrocomputer 9 and processed. When the switching valve 6 is opened, theexhaust gas does not flow into the NO_(x) reducing catalyst 4, but flowsonly into the three way catalyst.

Embodiment 6

Furthermore, another embodiment of the present invention is shown inFIG. 19. In this case, a sensor 73 as shown in FIG. 16(A) is arranged atupstream side of a switching valve 6. The switching valve 6 is operatedby a driving apparatus 74 which is controlled by signals from amicrocomputer 9. Exhaust gas before and after a catalyst 4 is introducedinto the sensor 73. The switching valve is arranged at downstream sideof the sensor 73 so that the exhaust gas at downstream side of thecatalyst 4 does not flow into a side of the sensor 73 for the exhaustgas at upstream side of the catalyst 4.

Embodiment 7

Furthermore, another embodiment of the present invention is shown inFIG. 20. Under a lean operating condition, a switching valve 75 isopened so that exhaust gas flows into a NO_(x) reducing catalyst 4. Inthis case, exhaust gas at downstream side of the catalyst 4 flows to aside of a sensor 78 for the exhaust gas at downstream side of thecatalyst 4. However, because a small amount of gas leaks through theswitching valve 75 to an exhaust pipe 76, the exhaust gas at upstreamside of the catalyst 4 flows to a side of a sensor 78 for the exhaustgas at upstream side of the catalyst 4. Under an operating conditionother than the lean operating condition, the switching valve 75 isswitched in a manner as shown by a dotted line in FIG. 20 so that theexhaust gas flows to an exhaust pipe 76. A three way catalyst 5 isarranged in the exhaust pipe 76. In accordance with the structuredescribed above, deterioration of the catalyst can be detected by only asensor.

Embodiment 8

Next, an embodiment of the present invention relating to an enginecontrolling method for detecting deterioration of catalyst andefficiency of the engine, and subsequently for improving the efficiency.

FIG. 21 is a simplified diagrammatic view of a total system used for theabove engine controlling method. Sensors 7, 8 are provided before andafter a catalyst 4. A sensor 80 for detecting gas temperature isprovided at an exhaust pipe. It is necessary to control catalysttemperature and HC concentration so as to obtain maximum efficiency ofthe catalyst as shown in FIG. 2 after judging deterioration of thecatalyst based on outputs from the sensors 7, 8.

Methods for controlling the catalyst temperature and the HCconcentration are explained hereinafter.

One of the methods for controlling the catalyst temperature is a methodfor regulating circulation of cooling water 86 in engine 1. Whencirculating amount of the cooling water is decreased by regulating acontrol valve 87, combustion temperature of the engine 1 is elevatedand, accordingly, exhaust gas temperature is also elevated. That means,when the catalyst is judged as deteriorated, the circulating amount ofcooling water is regulated to elevate the exhaust gas temperature so asto prevent lowering the efficiency of the catalyst. Another method forcontrolling temperature is a method for regulating ignition timing of anignition apparatus 84 and an ignition plug 20. Delayed ignition timingelevates the exhaust gas temperature. That means, when the catalyst isdeteriorated, the ignition timing is delayed to elevate the exhaust gastemperature so as to prevent lowering the efficiency of the catalyst.Further, because a required amount of HC changes depending ondeterioration of the catalyst, the amount of HC must be increased whenthe catalyst is deteriorated.

Next, methods for increasing the amount of HC are explained hereinafter.

An exhausted amount of HC changes in accordance with change of fuelinjection timing of an injection valve 19. Therefore, when the catalystis deteriorated, a setting of fuel injection timing is changed by acontroller 9. Further, when the injection valve 19 is an air assistinjection valve which atomizes fuel by a jet stream, an exhausted amountof HC increases by reducing or stopping of the air assist because fuelatomization becomes incomplete. When a flow dividing valve 18 is closedso that intake gas flows through a swirl path 17, swirling currents aregenerated in a combustion chamber and combustion is improved. Therefore,on the contrary, an amount of air flow flowed through paths other thanthe swirl path is increased by making the flow dividing valve 18 a halfopen, and combustion of fuel becomes incomplete. Accordingly, anexhausted amount of HC is increased. Furthermore, cooling of the engineby increasing circulating amount of cooling water increases theexhausted amount of HC. Further, advanced ignition timing increases theexhausted amount of HC. Therefore, when the catalyst is deteriorated,the ignition timing may be advanced depending on a deteriorating degreeof the catalyst. On the contrary, the catalyst temperature and theexhausted amount of HC can be increased by operation at the exhaustedgas side. Vaporized fuel components stored at upper portion of inside afuel tank 82 are supplied by pumping to inside the exhaust pipe atupstream side of the catalyst. As the vaporized fuel components are HC,the efficiency of the catalyst can be improved by supplying thevaporized fuel components to the catalyst. Furthermore, the vaporizedfuel components are combustible easily because they are light HC, andthe catalyst temperature is elevated by combustion of the vaporized fuelcomponents.

Another method for elevating the catalyst temperature uses an electricheater 88 for heating the catalyst. A driving circuit 81 supplies powerto the heater 88 in accordance with signals from the controller 9. Theheater elevates the catalyst temperature, and the efficiency of thecatalyst is improved. As for the heater, the one, wherein carriers ofthe catalyst are made from conductive materials and power is supplied tothe carrier, may be usable.

Another method for altering the catalyst temperature is indicated inFIG. 22. In this method, a structure wherein a burner is installed atinside the exhaust pipe 3 is adopted, and sparks are formed by anignition plug 91 to which electric voltage is supplied from a drivingcircuit 81 for ignition apparatus. Fuel is injected into the burner 93from a fuel injector 92. Air is supplied to the burner 93 by a pump 94.The fuel injected into a chamber 93 is ignited by the ignition plug 91,and flame is formed inside of the exhaust pipe before the catalyst. Thecatalyst temperature increases by the flame. A structure having anelectric heater 88 is also indicated in FIG. 22. The heater 88 is socomposed as to wrap outside of the catalyst 4 in order to heat thecatalyst effectively. Further, each of the above described methods canbe used in a combined manner or independently. That means, any of theabove described methods can operate independently.

A flow chart for control is shown in FIG. 23. When deterioration ofcatalyst is detected at step 710, a judgement whether any changing in anamount of HC is necessary or not is performed based on deterioratingdegree of the catalyst at step 720. When the changing in the amount ofHC is judged as necessary, an amount of HC to be changed is determineddepending on the deteriorating degree of the catalyst at step 730.Subsequently, an amount of HC supply to the catalyst is changed by anyone or any combination of HC increasing or decreasing means shown inFIGS. 21 and 22 at step 740. Generally speaking, when NO_(x) reducingcatalyst is deteriorated, HC supply to the catalyst must be increasedfor ensuring its efficiency. Further, when deterioration of the catalystis detected at step 710, its deteriorating degree is indicated at step750.

A flow chart for changing catalyst temperature is shown in FIG. 24. Whendeterioration of catalyst is detected at step 800, its deterioratingdegree is indicated, and a judgement whether any changing in anoperating temperature of the catalyst is necessary or not is performed.When the changing is judged as necessary, a necessary changing degree ofthe operating temperature is decided at step 820, and the catalysttemperature is changed by any one or any combination of temperaturechanging means shown in FIGS. 21 and 22 at step 830. Subsequently, thecatalyst temperature is judged whether it becomes a designatedtemperature or not, and when it is judged as the designated temperature,the flow of processing is finished.

A flow chart for controlling operating conditions so as to maintain theconversion efficiency of catalyst always at maximum is shown in FIG. 25.First, the present operating condition is judged whether within a leanoperating region or not at step 900, and when within the lean operatingregion, it is judged whether in a normal operation or not. When it is inthe normal operation within the lean operation region, signals fromsensors for measuring efficiency of the catalyst are taken at step 920,and the conversion efficiency of the catalyst is estimated at step 930.When the conversion efficiency of the catalyst is judged as lower than adesignated value at step 940, parameters such as catalyst temperatureand supplied amount of HC are changed by controlling an efficiencychanging means at step 950. Subsequently, the conversion efficiency ofthe catalyst is judged whether it is improved or not. When it is judgedas being improved, the flow of processing is finished in keeping withthe changed parameters, and when it is judged as being not improved, theflow of the processing is finished after returning the parameters to thevalues before changing at the step 970. In accordance with the abovedescribed method, an engine can be operated always in a condition withpreferable conversion efficiency.

Another method for detecting efficiency and deteriorating degree isindicated in FIGS. 26(A), 26(B), and 26(C). In this method, fluctuationwidth ΔV of output signals from sensors 7, 8 before and after catalyst 4as shown in FIG. 26(A) are compared each other. When oxygenconcentration in upstream to the catalyst 4 is fluctuating, state of thefluctuation can be determined by the fluctuation width ΔV1 of outputsignal from sensor 7 as shown in FIGS. 26(B) and 26(C). While thecatalyst is new, oxygen concentration in downstream from the catalyst 4fluctuates significantly because of its active reactivity for NO_(x)reducing reaction. However, as the catalyst being deteriorated, itsreactivity for NO_(x) reducing reaction decreases and the fluctuation ofthe oxygen concentration becomes less. Therefore, the fluctuation widthΔV2 of output signal from sensor 8 in downstream after the catalyst 4becomes small. Accordingly, a deteriorating degree of the catalyst canbe determined by measuring changing of the fluctuation width. A flowchart for detecting the deteriorating degree is shown in FIG. 27.Operating condition is judged at step 1000 whether it is within a leanoperation region or not, and when the operating condition is judged aswithin the lean operating region and the catalyst is in a designatedtemperature range at step 1010, ΔV1 and ΔV2 are detected and calculatedat step 1020. Subsequently, ΔV2/ΔV1 is calculated at step 1030. Next,the ΔV2/ΔV1 is judged whether it is smaller than a designated value ornot. When it is small, efficiency of the catalyst is judged as decreasedat step 1040, the deteriorating degree is indicated at step 1050, and anefficiency improving means starts its operation at step 1060. When onlyjudging the efficiency of the catalyst is required, starting andcontrolling the efficiency improving means can be omitted.

Furthermore, another method for detecting efficiency and deterioratingdegree is indicated in FIGS. 28(A), 28(B), and 28(C). Referring to FIG.28(A), an air-fuel ratio of mixture which is supplied to an engine 1 isaltered intentionally by changing injecting amount of fuel and throttleopening of a fuel injecting valve 19, and the deteriorating degree ofthe catalyst is determined based on difference in behavior of signalsfrom the sensors 7, 8, which are located at upstream side and downstreamside of the catalyst 4, respectively. As shown in FIG. 28(B), when theair-fuel ratio is altered stepwise, outputs from the sensors 7, 8 alsochange stepwise. However, when the catalyst is deteriorated, adifference in responses from the respective sensors 7, 8 located atupstream side and downstream side of the catalyst changes from thedifference when the catalyst is not deteriorated. For instance, behaviorof sensors which have linear outputs corresponding to the air-fuel ratioare indicated by solid lines in FIG. 28(B), in which the difference intime constant τ of the response increases when the catalyst isdeteriorated. Behavior of regular oxygen sensor outputs are alsoindicated by chain lines, in which the difference in time constant τ ofthe response increases similarly when the catalyst is deteriorated. InFIG. 28(C), a case when the air-fuel ratio of the mixture at supply sideis altered randomly or in accordance with a rule is indicated. Thedeteriorating degree is determined by correlation between changes insignals from the sensors 7, 8. When the signal from the sensor 8 isremarkable more dull than that from the sensor 7, the catalyst can beregarded as deteriorated. A flow chart for determining the deteriorationof the catalyst by the present method is indicated in FIG. 29. Afteroperating condition is judged at step 1100 as within a lean operatingregion and in a normal operation, catalyst temperature is judged whetherit is in a designated temperature range or not at step 1110.Subsequently, after satisfying the step 1110, air-fuel ratio of mixturesupplied to the engine is altered at step 1120. Next, the deterioratingdegree of the catalyst is determined by the method indicated in FIGS.28(A), 28(B), and 28(C) based on the difference of behavior in timeconstant τ of the response signal V1, V2 from the sensors before andafter the catalyst at step 1130. When it is judged as deteriorated instep 1140, the deteriorating degree is indicated at step 1150, and,subsequently, operation of the efficiency improving means shown in FIGS.21 and 22 is started at step 1160.

In accordance with the present invention, it becomes possible to detectdecrease in catalyst efficiency on account of long period operationaccurately. Furthermore, an engine can be controlled so as to avoiddecreasing in the efficiency, and a preferable exhaust gas cleaningcharacteristics can be maintained.

What is claimed is:
 1. An efficiency controlling method for an NOx⁻eliminating catalyst installed in an exhaust gas system of an internalcombustion engine, comprising (a) evaluating said NOx⁻ eliminatingcatalyst, and (b) increasing an amount of hydrocarbon supplied to saidNOx⁻ eliminating catalyst when said NOx⁻ eliminating catalyst is judgedas deteriorated to increase efficiency of the catalyst.
 2. An efficiencycontrolling method for an NOx⁻ eliminating catalyst installed in anexhaust gas system of an internal combustion engine, comprising (a)evaluating said NOx⁻ eliminating catalyst, (b) elevating a temperatureof said NOx⁻ eliminating catalyst, and optionally (c) increasing anamount of hydrocarbon supplied to said NOx⁻ eliminating catalyst whensaid NOx⁻ eliminating catalyst is judged as deteriorated to increaseefficiency of the catalyst.
 3. A method for controlling NOx catalyst,comprising controlling one of temperature of exhaust gas and hydrocarbon(HC) concentration to increase efficiency of the catalyst under adeteriorated condition high when deterioration of the catalyst isdetermined.
 4. A method for controlling NOx catalyst according to claim3, wherein said temperature of exhaust gas is controlled by controllingignition timing.
 5. A method for controlling NOx catalyst according toclaim 3, wherein said HC concentration is controlled by controlling fuelinjection timing.
 6. A method for controlling NOx catalyst according toclaim 3, wherein either of said temperature of exhaust gas or said HCconcentration is controlled by increasing its temperature orconcentration.